The present invention relates to sorting integrated circuit dice created on wafers. In particular, the present invention relates to setting limits for acceptable quiescent currents on a wafer lot basis or a wafer basis to sort and reject dice.
Integrated circuits are typically produced on large circular wafers of silicon. Each wafer is divided into a matrix of dice or chips with each die containing one complete integrated circuit. The wafers are produced in a batch process such that each wafer is associated with a wafer lot typically containing 24-48 wafers.
The production techniques used to produce the dice are not perfect and thus some dice are defective. To avoid shipping defective dice to customers, the dice are tested. One series of tests, known as wafer sort, is performed after the circuits have been completely formed on the wafer but before the individual dice have been packaged for shipment. A second series of tests is performed after the individual dice have been assembled into die packages that are later used to connect the dice to other circuit elements. This final series of tests is known as final test.
One type of test that is performed during both wafer sort and final test is known as a quiescent current test. The quiescent current test measures the amount of current drawn through the power inputs of each die while the inputs of each die are placed in a state that turns off all of the active devices in the die. In the past, quiescent current testing was performed by setting an acceptable quiescent current test limit based upon the dice circuit design. In the prior art, any dice with a quiescent current above the test limit were rejected. This quiescent current test limit was set once and was not changed until the circuit design changed.
This fixed limit has generally been set to maximize the number of dice that are shipped for each wafer lot. However, this yield maximization ignores differences between dice in a lot. Thus, even though all of the dice that are shipped are operational at ship time, there may be some dice that are substantially different from the remaining dice in their respective lots.
The present invention recognizes that these substantially different dice, even though operational at ship time, are likely to fail or cause other problems after being shipped. Such "maverick" or abnormal dice could be rejected before shipment by lowering the fixed quiescent current limit. However, lowering the limit results in a significant decrease in yield since the lower limit causes large numbers of non-maverick dice to be rejected from some lots. Thus, a new method for determining a quiescent current test limit is needed to avoid shipping dice with abnormal quiescent currents.